1. Field of the Invention
The present invention relates to a DSS (Distributed Sample Scrambling) system for scrambling and descrambling, and more particularly to a sample value extraction apparatus in the descrambling part which extracts two sample values from received data in order to synchronize PRBS data (Pseudo Random Binary Sequence) which are in the scrambler and descrambler each other.
2. Description of the Conventional Art
Generally, the DSS system is adopted for use in the cell-based physical layer of BISDN (Broadband Integrated Service Digital Network) by CCITT (International Telegraph and Telephone Consultative Committee). This system is composed of a scrambler and a descrambler, randomizing data. Therefore, in the scrambler, The PRBS data is generated and exclusively ORed with the data inputted. In addition, two sample values are latched from the PRBS data at a uniform interval. That is, One sample value is latched at the uniform interval too. And these two latched sample values are transmitted via scrambled data, inserting them in the HEC8 and HEC7 each.
In the descrambler, using the transmitted sample values, the descrambler PRBS data is corrected and become identical to the scrambler PRBS data state. Therefore this descrambler PRBS data is added to the scrambled data and the descrambled data is obtained.
For example, in the case of the scrambling of the transmitter, if an input data is formed of "111110101", and the scrambler PRBS data is formed of "110101000", then the scrambled data is formed of "001011101".
In addition, in the case of the descrambling of the receiver, if the input data that is, received data is formed of "001011101", and the descrambler PRBS data is formed of "110101000", then the descrambled data is formed of "111110101" for thereby performing a binary arithmetic operation of 0 or 1 in accordance with the resultant data.
In the above-described DSS operation, PRBS data of the scrambler and the descrambler is generated, based on a creative polynomial expression of X.sup.31 +X.sup.28 +1, and the scrambling range is a header (4 bytes) and a payload (48 bytes) except for an HEC (1 byte).
In the sample latch method of transmitter, the sample value is latched from the PRBS data of the scrambler at every 212 bit through a 212 counter. And two sample values are inserted into the scrambled data HEC8 and HEC7.
As shown in FIG. 1, the conventional transmitter includes a scrambler 1 having a PRBS data generator 2 for generating a PRBS data, and an exclusive OR-gate 3 for exclusively ORing the PRBS data from the PRBS data generator 2 and an input data from the exclusive OR-gate 3, a HEC encoder 4 for encoding an output signal from the scrambler 1, and a sample value latching unit 6 for latching the sample value from the PRBS data generator 2, and an exclusive OR-gate 5 for exclusively ORing an output signal from the HEC encoder 4 and a sample value latched from the PRBS data generator 2.
FIG. 2 illustrates a part of PRBS data from the PRBS data generator 2. As shown in FIG. 1, the scrambler 1 performs a scrambling operation through entire input cell data interval except for an HEC 8-bit interval (HEC1-HEC8), as shown in FIG. 3.
The scrambled data are CRC-computed with respect to 32 bits from a start point of a cell by the HEC encoder 4, and the 8 bits which are the resultant value is inserted into the HEC 8 bit interval.
Next, two sample values (U.sub.t+1 and U.sub.t-211), which are obtained by the sample value latching unit 6 by latching the PRBS data from the scrambler 1 by one bit at the interval of 212 bits are exclusively ORed in the HEC eighteenth interval and the HEC seventh interval for thereby transmitting the data.
Namely, two sample data are latched by the one cell which is formed of 424 bits, and such the latched two sample data are exclusively ORed for thereby generating a transmission data as shown in FIG. 3. Here, U.sub.t represents the PRBS data generated at a current time, U.sub.t+1 represents data after U.sub.t, U.sub.t-211 represents data which is 211st earlier than U.sub.t, and HEC8, HEC7, . . . , HEC1 are all 8-bits which are generated by CRC-computing 32-bit, namely, 4-byte.
Therefore, the PRBS data of the scrambler and the input data are exclusively ORed from the start point of the cell by one bit, and in the HEC interval, two sample values are exclusively ORed after the HEC 8-bit signal which is obtained by CRC-computing 32-bit data is inputted.
However, in the conventional art, the sample value from the transmitter is included in the receiving data. In addition, a method for extracting the above-described values in the descrambler is not disclosed.
In the descrambler of DSS system, key issue is to generate the PRBS data, which is identical to the PRBS data of the scrambler. So first, it is need to extract the sample values, which are transmitted via the input data.
Therefore, this invention is derived to resolve the above disadvantages and presents a method of catching the sample values in the descrambler in order to gain synchronization for the scrambler PRBS data and the descrambler PRBS data.